Free fluid and no mass

Hi,

I have had digestive problems and am going through a bunch of "rule outs." I'm post menopausal by about a year and a half. They saw a pelvic mass, about two inches near the area of my uterus.. When they did the ultrasound, there was no mass. I have a retroverted uterus. That's what the gastroenterologist thinks it is. But there was a "moderate" amount of free pelvic fluid. What could that be?

Thanks!

Elizabeth